The Cancer Research UK "Over 50s" trial compared 5 and 2 years of tamoxifen in women with early breast cancer. Results are reported after median follow-up of 10 years. Between 1987 and 1997, 3,449 patients age 50 to 81 years with operable breast cancer who had been taking 20 mg of tamoxifen for 2 years were randomly assigned to either stop or continue for an additional 3 years, if they were alive and recurrence free. Data on recurrences, new tumors, deaths, and cardiovascular events were obtained (April 2010). There were 1,103 recurrences, 755 deaths as a result of breast cancer, 621 cardiovascular (CV) events, and 236 deaths as a result of CV events. Fifteen years after starting treatment, for every 100 women who received tamoxifen for 5 years, 5.8 fewer experienced recurrence, compared with those who received tamoxifen for 2 years. The risk of contralateral breast cancer was significantly reduced (hazard ratio, 0.70; 95% CI, 0.48 to 1.00). Among women age 50 to 59 years, there was a 35% reduction in CV events (P = .005) and 59% reduction in death as a result of a CV event (P = .02); in older women, the effect was much smaller and not statistically significant. Taking tamoxifen for the recommended 5 years reduces the risk of recurrence or contralateral breast cancer 15 years after starting treatment. It also lowers the risk of CV disease and death as a result of a CV event, particularly among those age 50 to 59 years. Women should therefore be encouraged to complete the full course. Although aromatase inhibitors improve disease-free survival, tamoxifen remains a cheap and highly effective alternative, particularly in developing countries.